1. Field of the Invention
The present invention relates to an electrode contact structure, particularly to a contact structure between an Au electrode and an Al wiring and a method for fabricating the same.
2. Related Art
As a material for an ohmic electrode coming in contact with a compound semiconductor used as an optical element of a light-emitting or light-receiving element, an alloy consisting essentially of Au is usually used. For example, AuGe, AuIn, AuSi and AuSn are used for a p-type electrode, and AuZn, AuMo, AuIn and AuBe for an n-type electrode. As a material for a wiring, Al is usually used, which has a low resistivity and is suitable for wire bonding.
However, in the combination of an alloy consisting essentially of Au and aluminum, Au—Al intermetallic compound are formed due to a counter diffusion of Au atoms and Al atoms in a contact hole opened in an SiO2 insulating film in which an Au alloy and Al are ohmically contacted to each other. As Au—Al intermetallic compound, five kinds of compounds are known such as Au4Al, Au5Al2, Au2Al, AuAl and AuAl2 that are alloys of metastable composition. Au5Al2 has a specific volume which is three to four times a volume of sole Au or Al. That is, Au5Al2 is expanded three to four times in volume, so that stress is stored in a contact hole. As a result, an Al wiring and/or an insulating film are/is broken away to cause the breakage of a wiring, which has a bad influence upon the reliability of an electrode contact structure. Such an intermetallic compound Au5Al2 is formed at the peripheral portion of a contact hole, i.e., at the portion being in contact with an SiO2 insulating film.
In the case that there are many contact holes in an array structure including a plurality of optical elements, especially in an array comprising a driver circuit such as a self-scanning light-emitting element array using three-terminal light-emitting thyristors of PNPN structure (see Japanese Patent No. 2683781), if the intermetallic compound Au5Al2 is formed, a plurality of contact holes, the number thereof being 3-10 times the number of light-emitting elements, the integrity of at least one contact hole is compromised.